Specific Aims: The purpose of the present proposal is to study the degree of functional synchronization that exists within the phrenic motoneuron pool. Much experimental evidence indicates that this important motor nucleus behaves not merely as a simple relay station, interfacing descending respiratory drives with the diaphragm. Rather, a substantial amount of information processing is occurring in the spinal cord itself. We plan to address several pertinent questions: 1) What types of phrenic motoneurons are synchronized on a short time scale basis? 2) What is the influence of intercellular spacing on unit-to-unit synchronization? 3) Are descending inputs from the medulla distributed preferentially to specific phrenic motoneuronal types? 4) What is the impact of dual versus single input drive activation of phrenic cells on mechanisms of synchronization? Methodology: The primary method to be employed is that of cross-correlation analysis on phrenic motoneuron pairs. The animal model will be the decerebrate, immobilized, and ventilated cat. Two metal microelectrodes will be inserted into the phrenic nucleus to record extracellur potential activities from single phrenic cells in the cervical spinal cord. The distance between recording electrodes will be known and varied throughout the entire phrenic nucleus (20 mm long). Idealized spike activities will be recorded on magnetic tape for off-line computer analysis. Two series of cats will be studied: one in which dual descending drives impinge upon the phrenic nucleus, and another in which the high frequency input is blocked with pentobarbital. Long-Term Objectives: The significance of this type of research complements the classical work on the size principle of motoneurons which helps to explain recruitment order of cells. The overall goal will be to substantiate (or refute) the hypothesis that phrenic motoneurons are not locked into stereotyped discharge patterns as demanded by the size principle. Also, future experiments may elucidate the source of separate medullary drives descending onto phrenic motoneurons. Health-Relatedness: There is a large interest today on the control of muscle function in man, especially in paraplegics and quadriplegics. Because the respiratory system is subjected to natural, rhythmic inputs, it becomes an excellent model for the study of motor systems in general. The close examination of synchronization mechanisms in the phrenic nucleus, therefore, has ramifications to other motoneuronal systems where synchronization is also of crucial importance.